1. Field of Invention
The invention relates to a fracture analysis method for a spot weld portion of three or more mutually superimposed plates that are spot-welded at a common welding point and a computer-readable program for executing the method as well as a spot weld fracture analysis apparatus.
2. Description of Related Art
There is a known fatigue life prediction method for a spot-welded structure characterized in that two plates are joined to form a spot-welded structure, and that a finite element analysis-purpose shell model for the spot-welded structure is created, and that using the created purpose shell model, a finite element linear elastic analysis is performed to calculate the share load of a nugget portion at the center of a spot weld portion, and the deflection on the circumference of a circle of a diameter D described with the nugget portion being the center, and the inclination in radial directions, and that, on the basis of the share load, the deflection on the circumference and the inclination in radial directions that have been calculated, the nominal structural stress in the nugget portion is found through the use of a disc bending theory in the theory of elasticity, and that, from the nominal structural stress, a fatigue life of the spot-welded structure is predicted (e.g., see Japanese Patent Application Laid-Open Publication No. 2003-149130).
A spot weld portion of three or more mutually superposed plates that are spot-welded at a common welding point can be modeled, for example, by modeling each plate by shell elements, and connecting the shell elements corresponding to the spot welding point positions of the individual plates via beam elements. Such a finite element model is substantially equal to a mode in which two plates are respectively spot-welded to each other at a time. That is, for example, in the case of three-plate welding, a mode is obtained in which the middle plate is spot-welded separately to the two side plates.
Consequently, in the case where such a finite element model is used, a fracture determination method whose conformity with regard to the spot welding of two plates has been established can be utilized for every group that is made up of two plates.
However, in the case where the fracture analysis method is directly utilized, the fracture analysis on a group does not take into account the element force of the beam element in another group. Therefore, a problem is that the possibility of fracture of the spot weld portion on a middle plate belonging to two groups (i.e., a middle plate that forms groups with two plates (inside and outside plates)) cannot be appropriately analyzed or predicted.